1. Field of the Invention
The present invention relates to an image pickup lens and a design method thereof and particularly, to an image pickup lens comprising one lens which can be reduced in size and weight while achieving a wider angle of view to be used for an image pickup device (e.g., a CCD camera) utilizing an image pickup element such as a CCD, a CMOS or the like to be mounted on a portable computer, a visual telephone, a cellular phone and the like, and a design method thereof.
2. Description of the Related Art
Recently, there is a remarkable development in the multimedia industry. For example, there has been an increasing demand for a camera (e.g., a CCD camera) utilizing an image pickup element such as a CCD, a CMOS or the like to be mounted on a portable computer, a visual telephone, a cellular phone and the like. Such CCD camera needs to be mounted on a limited space. Thus, it is desirable that the camera be small in size and light in weight.
Accordingly, an image pickup lens used for such CCD camera is also required to be small and lightweight as well.
Conventionally, the so-called one-lens system using a single lens is used as such image pickup lens.
FIG. 1 shows a conventional image pickup lens with a one-lens system. The one-lens system comprises a lens body 10 having a positive power and a diaphragm 11 is disposed on an object side of the lens body 10. Further, on the image plane side of the lens body 10, a first cover glass 12, a second cover glass 13 and an image pickup surface 14 as a light receiving surface for an image pickup element such as a CCD, a CMOS or the like are disposed in order. Each lens face of the lens body is referred to be the first face and the second face, respectively, in order from the object side.
The image pickup lens is set under the following condition:
f=4.10 mm, F No=2.2, 2xcfx89=60.0xc2x0 Petzval sum=0.681
where, f denotes the focal length (mm) of the whole system, F No denotes F number, and 2xcfx89 denotes the maximum angle of view. Also, r denotes the radius of curvature (mm) of the lens and the like, d denotes the distance (mm) between each optical face, nd denotes the refractive index of an optical material (medium) present between with the next optical face, and xcexdd denotes the Abbe constant.
Provided the optical axis direction is taken as a Z-axis, the direction perpendicular to the optical axis is taken as an X-axis, and the traveling direction of light is defined to be positive, the shape of the aspherical face of the lens is expressed by a following expression (Eq1):   Z  =                              x          2                r                    1        +                              1            -                                          (                                  k                  +                  1                                )                            ⁢                                                x                  2                                                  r                  2                                                                          +                  a                  4          ⁢                      xe2x80x83                              ⁢              x        4              +                  a                  6          ⁢                      xe2x80x83                              ⁢              x        6            
where, each of k, a4, a6 is an aspherical factor.
However, in the one-lens image pickup lens of the related art, the Petzval sum and the curvature of field are large so that an excellent image plane cannot be obtained. Also, each aberration such as the longitudinal chromatic aberration, the chromatic aberration of magnification, the spherical aberration, the astigmatism is large so that each aberration cannot be corrected appropriately. Therefore, it is not possible to obtain an excellent optical characteristic.
FIG. 2 shows the chromatic aberration of magnification of ray paths A, B, and C in regards to the image pickup lens shown in FIG. 1. It shows that the chromatic aberration of magnification is large in the related art and it becomes large especially in the vicinity of the image pickup lens, thereby resulting in deterioration of the lens property. FIG. 3 shows the spherical aberration, the astigmatism, and the distortion aberration of the image pickup lens. It shows that each of the spherical aberration, the astigmatism, and the distortion aberration is large and, particularly, the spherical aberration and the stigmatism are large. Thus, it is clear that a sufficient optical characteristic cannot be obtained.
In view of this respect, there is a technique of the related art as disclosed in Japanese Patent Application Laid-open No. 10-73760 in which a diffraction element is unified with a refractive lens so as to correct the chromatic aberration of the image pickup lens by the diffraction element.
However, with this lens, the Petzval sum cannot be corrected thereby causing a large curvature of field. The reason is that there is no contribution by the diffraction element imposed onto the Petzval sum so that it is determined only by the property of a refractive lens.
The invention has been designed to overcome the foregoing problems. It is an object of the present invention to provide an image pickup lens and a design method thereof, which can achieve minimization of the system and a remarkable improvement in the optical characteristic with a simple structure.
In order to achieve the aforementioned objects, an image pickup lens of the present invention comprises a lens body having a second face on an image pickup surface side of the lens body being formed into a Fresnel face and a diffraction element being unified with at least either a first face of on an object side or the second face on the image pickup surface side of the lens body.
In the invention constituted as described, the second face on the image pickup surface side of the lens body is formed into a Fresnel face. Thus, it becomes possible to correct the Petzval sum and reduce the curvature of the field so that an excellent image plane can be obtained. Also, the diffraction element is unified at least with either the first face on the object side or the second face on the image pickup surface side of the lens body. Therefore, due to a color dispersion characteristic of the diffraction element, the chromatic aberration can be well corrected. Thereby, the optical characteristic of the image pickup lens can be remarkably improved.
Also, the diffraction element may be unified with the second face of the lens body.
By unifying the diffraction element with the second face of the lens body, chromatic aberration can be well corrected and the optical characteristic of the image pickup lens can be remarkably improved due to a color dispersion characteristic of the diffraction element.
Further, the first face of the lens body may be formed into an aspheric face and the second face of the lens body may be formed into an aspheric Fresnel face.
By forming the first face of the lens body into an aspheric face and the second face of the lens body may into an aspheric Fresnel face, each aberration such as the spherical aberration and the like can be appropriately corrected. Thereby, the optical characteristic of the image pickup lens can be remarkably improved.
Furthermore, in the image pickup lens of the present invention, the lens body may be formed to satisfy a condition represented by a following expression:
1.25 less than d/f less than 1.45xe2x80x83xe2x80x83(1)
where,
d: thickness in the center of the lens body; and
f: focal length of the lens body.
By satisfying the expression (1), the distortion aberration amount can be corrected with a comfortable visual sense while well maintaining each aberration except for the distortion aberration. In the expression (1), if the d/f value is larger than 1.45, the distortion aberration becomes large in the positive direction so that the back focus becomes short. If the d/f value is smaller than 1.25, the distortion aberration in the negative direction becomes large so that, even though the distortion aberration can be suppressed, the curvature of field, the longitudinal chromatic aberration, the lateral aberration, the spherical aberration and the like cannot be well corrected.
Also, in the image pickup lens of the present invention, the lens body may be formed to satisfy a condition represented by a following expression:
0.6 less than |r2/r1| less than 0.75xe2x80x83xe2x80x83(2)
where,
r1: radius of curvature in the center of the first face of the lens body; and
r2: radius of curvature in the center of the second face of the lens body.
By satisfying the expression (2), an ideal lens shape in which each aberration is well balanced can be achieved. In the expression (2), if the |r2/r1| value is larger than 0.75 or smaller than 0.6, mainly the spherical aberration becomes worsen resulting in deterioration of resolution.
Further, in the image pickup lens of the present invention, a diaphragm may be positioned on the object side of the lens body and the lens body may be formed to satisfy a condition represented by a following expression:
0xe2x89xa6ds/fxe2x89xa60.4xe2x80x83xe2x80x83(3)
where,
ds: distance between the center of diaphragm and the first face of the lens body; and
f: focal length of the lens body.
By satisfying the expression (3), the coma aberration can be corrected while achieving an excellent balance of coma aberration and the distortion aberration. In the expression (3), if the ds/f value is larger than 0.4, the coma aberration cannot be sufficiently corrected and if it is smaller than 0, the diaphragm is to be in the lens body. Thus, it is not desirable since it becomes difficult to manufacture.
A design method of an image pickup lens according to the present invention is for determining the thickness and the radius of curvature in the center of a lens body having a second face on an image pickup surface side of the lens body being formed into a Fresnel face and a diffraction element being unified with at least either a first face on the object side or the second face on the image pickup surface side of the lens body. The design method comprises the steps of: under the condition that each aberration is maintained to be excellent except for distortion aberration, performing ray tracing simulation beforehand on a lens body group with a different combination of the d/f value (where, d denotes the thickness in the center of a lens body and f denotes the focal length of a lens body) and the |r2/r1| value (where, r1 denotes the radius of curvature in the center of the first face of the lens body and r2 denotes the radius of curvature in the center of the second face of the lens body); calculating the distortion aberration of the lens bodies so as to obtain dependency of the distortion aberration on the d/f value and the |r2/r1| value; selecting, according to the correlation, the combination of the d/f value and the |r2/r1| value to be a desired distortion aberration; and determining the thickness and the radius of curvature of the lens body on the basis of the d/f value and the |r2/r1| value.
In the method of the present invention as described, the thickness in the center of the lens body and the radius of the curvature in the center are determined based on the d/f value and |r2/r1| value obtained beforehand. Therefore, it becomes possible to design a lens body in which distortion aberration is appropriately corrected.
Further, in the image pickup lens of the present invention comprises a lens body having at least one face being formed into an aspherical face and at least either the first face on the object side of or the second face on the image pickup surface side of the lens body being formed into a Fresnel face, and the lens body may be formed to satisfy a condition represented by a following expression:
1.17 less than d/f less than 1.4xe2x80x83xe2x80x83(4)
where,
d: thickness in the center of the lens body; and
f: focal length of the lens body.
By forming at least one face of the lens body into an aspherical face and at least either the first face on the object side or the second face on the image pickup surface side of the lens body into a Fresnel face, it becomes possible to correct the Petzval sum and reduce the curvature of the field so that an excellent image plane can be obtained. Also, each aberration such as spherical aberration and the like can be well corrected. Thereby, the optical characteristic of the image pickup lens can be remarkably improved.
Also, by satisfying the expression (4), the distortion aberration amount with a comfortable visual sense can be achieved while well maintaining the aberration except for the distortion aberration. In the expression (4), if the d/f value is larger than 1.4, the distortion aberration becomes large in the positive direction so that the back focus becomes short. If the d/f value is smaller than 1.17, the distortion aberration in the negative direction becomes large so that, even though the distortion aberration can be suppressed, the curvature of field, the longitudinal chromatic aberration, the lateral aberration, the spherical aberration and the like cannot be well corrected.
Further, in the image pickup lens of the present invention, the lens body may be formed to satisfy a condition represented by a following expression:
0.58 less than |r2/r1| less than 0.73xe2x80x83xe2x80x83(5)
where,
r1: radius of curvature in the center of the first face of the lens body; and
r2: radius of curvature in the center of the second face of the lens body.
By satisfying the expression (5), an ideal lens shape can be achieved in which each aberration is well balanced. In the expression (5), if the |r2/r1| value is larger than 0.73 or smaller than 0.58, mainly the spherical aberration becomes worsen resulting in deterioration of resolution.
Furthermore, in the image pickup lens of the present invention, a diaphragm may be positioned on the object side of the lens body and the lens body may be formed to satisfy a condition represented by a following expression:
0xe2x89xa6ds/fxe2x89xa60.4xe2x80x83xe2x80x83(6)
where,
ds: distance between the center of diaphragm and the first face of the lens body; and
f: focal length of the lens body.
By satisfying the expression (6), the coma aberration can be corrected while achieving an excellent balance of coma aberration and the distortion aberration. In the expression (6), if the ds/f value is larger than 0.4, the coma aberration cannot be sufficiently corrected and if it is smaller than 0, the diaphragm is to be in the lens body. Thus, it is not desirable since it becomes difficult to manufacture.
Furthermore, a design method of an image pickup lens according to the present invention is for determining the thickness and the radius of curvature in the center of a lens body having at least the first face on the object side or the second face on the image pickup surface side of the lens body being formed into a Fresnel face. The design method comprises the steps of: under the condition that each aberration is maintained to be excellent except for the distortion aberration, performing ray tracing simulation beforehand on a lens body group with a different combination of d/f value (where, d denotes the thickness in the center of a lens body and f denotes the focal length of a lens body) and |r2/r1| value (where, r1 denotes the radius of curvature in the center of the first face of said lens body and r2 denotes the radius of curvature in the center of the second face of the lens body); calculating the distortion aberration of the lens bodies so as to obtain dependency of the distortion aberration on the d/f value and the |r2/r1| value; selecting, according to the correlation, the combination of the d/f value and the |r2/r1| value to be a desired distortion aberration; and determining the thickness and the radius of curvature of the lens body on the basis of the d/f value and the |r2/r1| value.
In the method of the present invention as described, the thickness in the center of the lens body and the radius of the curvature in the center are determined based on the d/f value and |r2/r1| value obtained beforehand. Therefore, it becomes possible to design a lens body in which distortion aberration is appropriately corrected.